Digital image sensing based upon solid state technology is well known, the two most common types of image sensors currently being charge coupled devices (CCD's) and complementary metal oxide semiconductor (CMOS) image sensors. Digital image sensors are incorporated within a wide variety of devices throughout the consumer, industrial and defense sectors, among others.
An image sensor is a device comprising one or more radiation sensitive elements having an electrical property that changes when radiation is incident thereupon, together with circuitry for converting the changed electrical property into a signal. As an example, an image sensor may comprise a photodetector that generates a charge when radiation is incident thereupon. The photodetector may be designed to be sensitive to electromagnetic radiation in the range of (human) visible wavelengths, or other neighboring wavelength ranges, such as infra red or ultra violet for example. Circuitry is provided that collects and carries the charge from the radiation sensitive element for conversion to a value representing the intensity of incident radiation.
Typically, more than one radiation sensitive element will be provided in an array. The term pixel is used as a shorthand for picture element. In the context of a digital image sensor, a pixel refers to that portion of the image sensor that contributes one value representative of the radiation intensity at that point on the array. These pixel values are combined to reproduce a scene that is to be imaged by the sensor. A plurality of pixel values can be referred to collectively as image data. Pixels are usually formed on and/or within a semiconductor substrate. In fact, the radiation sensitive element comprises only a part of the pixel, and only part of the pixel's surface area (the proportion of the pixel area that the radiation sensitive element takes up is known as the fill factor). Other parts of the pixel are taken up by metallization such as transistor gates and so on. Other image sensor components, such as readout electronics, analog to digital conversion circuitry and so on may be provided at least partially as part of each pixel, depending on the pixel architecture.
Image sensors of this type may be used for still image capture and for video capture. Even when an image sensor is primarily designed for still image capture, it is common for a video function to be provided for example to provide a viewfinder function in a screen of a digital camera.
One of the most important characteristics of any image sensor is its dynamic range, that is, the ratio between the minimum and the maximum signal that can be successfully reproduced by the image sensor. There are various fields in which a high or very high dynamic range is required. An example of this is the automotive field. It is known to provide sensors of various types at various locations in or on a vehicle such as an automobile. Image sensors may be provided to perform various functions based on detection of images inside the vehicle and outside the vehicle. Examples of functions relating to detection of images inside the vehicle include driver recognition (using facial recognition), driver drowsiness detection (using head detection and object tracking), and crash recorders. Examples of functions relating to detection of images outside the vehicle include automatic parking systems, lane change assistance systems, pre-crash detection, sign recognition, headlamp control. These lists of examples are of course non-exhaustive.
For all these sensors, a high dynamic range is important due to the wide variations in the scenes that are to be imaged. A common example is driving at night time. A sensor may be designed to detect the position of a line in the center of a road, and needs to be able to detect and provide data for tracking the position of the line, but the sensor's field of view will regularly encompass objects that are very much brighter than the line that it is designed to track, for example, the illumination provided by headlights of another vehicle driving in the opposite direction to the vehicle carrying the sensor. It is important that the sensor does not become saturated by the bright headlight illumination, but also retains the ability to keep tracking the line in the center of the road, that is, it requires a very high dynamic range in order to function correctly. Another common example is a vehicle that exits a tunnel during the daytime. For the example of a sensor designed to detect the position of a line in the center of a road, a sharp transition in brightness will occur as the vehicle moves between the relative darkness of the tunnel and brightness of the daytime environment outside of the tunnel.
In addition, motion artifacts can detract from the correct operation of a sensor. This is of particular concern in the automotive field as cars may be moving at high speeds in opposite lanes towards each other.
A high dynamic range is important for all sensors in the automotive field (for functions relating to detection of images inside and outside the vehicle), and in general to image sensors in other application areas such as machine vision and consumer camera devices for example.
It is known to augment the dynamic range of an image sensor by capturing multiple images with different exposures, and then combining the data from the multiple images, for example by summing weighted pixel values, to obtain a composite image. However, these techniques are memory intensive and require that the number of pixel readouts carried out is multiplied by the number of images taken.